One class of spray gun uses pressurized air for atomizing liquid such as paint and for shaping the envelope or pattern of the atomized liquid as it is discharged from a nozzle assembly on the gun. Air atomization spray guns broadly fall into two classes. One type of air atomization spray gun uses a low volume flow of high pressure air for atomization and pattern shaping. The air pressure typically may be on the order of from 40 psi (2.81 Kg/cm.sup.2) to as high as 100 psi (7.03 Kg/cm.sup.2), or more. This type of spray gun is capable of producing very fine uniform droplets of paint. However, the high pressure air discharged from the nozzle assembly imparts a relatively high velocity to the paint droplets. As a consequence of the high velocity, some of the droplets will be deflected from the surface being coated, producing a less than optimal transfer efficiency. Paint droplets that are not deposited on the surface being coated end up in the surrounding environment and consequently can present an environmental problem.
A second type of air atomization paint spray gun uses a relatively high volume flow of low pressure air for atomization and pattern shaping. The lower air pressure imparts a lower velocity to the atomized paint. However, the lower air pressure has resulted in larger paint droplets than produced with high air pressure and has presented problems in achieving uniformity in droplet size and optimal pattern shape for high production applications. The larger lower velocity droplets are less prone to be deflected from the surface being coated. Consequently, the transfer efficiency is increased and less paint is dispersed into the environment.
The pressure of the atomization and pattern shaping air used for HVLP spay guns is generally less than about 15 psi (1.05 Kg/cm.sup.2) and in some jurisdictions is kept to less than 10 psi (0.703 Kg/cm.sup.2). Some jurisdictions, for example, provide more lax air pollution control regulations if the air discharge pressure at the nozzle is no greater than 10 psi (0.703 Kg/cm.sup.2). The low air pressure may be produced either through the use of a high volume low pressure air turbine or by using a conventional high pressure compressed air source and suitable means for lowering the air pressure and increasing the volume flow, such as calibrated pressure dropping orifices or a pressure regulator.
An HVLP paint spray gun uses a nozzle assembly for fluid atomization and for shaping the pattern of the atomized fluid. The nozzle assembly includes a fluid tip and an air cap and also may include a baffle which distributes air flow. The fluid tip has a central fluid chamber leading to a fluid discharge orifice. A valve needle controls the discharge of fluid from the orifice. The air cap surrounds the fluid tip. An annular atomization air discharge orifice is formed between the fluid tip and the air cap. The air cap also typically includes two projecting horns on diametrically opposite sides of the fluid orifice for directing pattern shaping air at the atomized fluid to flatten the atomized fluid envelope. Prior art HVLP paint spray guns typically have a cylindrical projection on the fluid tip which extends at least flush with the front of the air discharge orifice in the air cap. The fluid discharge orifice is located at the front of the cylindrical projection. The cylindrical projection has been found to limit energy transfer to the fluid as it is emitted from the orifice and therefore to reduce the atomization efficiency. Another early fluid tip design had a conical exterior surface which projected through the air cap atomization air orifice. Previous HVLP nozzle assembly designs have not aggressively controlled air and fluid flow for maximum efficiency while maintaining high atomization performance. Consequently, prior art HVLP paint spray guns have had some deficiency in performance. The quality of atomization, particle size and pattern shape have not been as good as with high pressure atomization spray guns under certain spraying conditions. HVLP spray guns also have had difficulty in atomizing certain paints and in operating in high productivity situations while producing the highest quality finish coating.